The growing demands for portable electronic equipment and silicon-on-chip (SOC) products have been pushing the industry to design circuits with low power supply voltage, low power consumption, and smaller chip area. To meet this growing demand, integrated tunable filters have recently received great attention.
Tunable continuous time filters can be used for a variety of applications such as, for example, to remove aliasing and reduce in-band noise in various analog front-end systems, as realization of bandpass filters, as VCOs (voltage controlled oscillators), as loop filters for PLLs (phase-locked loops), and other such applications. Because of their efficient operation at high frequencies, and their easy integration with prevailing circuit fabrication technologies, transconductance-capacitor or gm-C filters have become very popular.
Transconductance elements form the building blocks for most gm-C filters. Currently, there exists several well known realizations for conductive metal oxide semiconductor (CMOS) transconductors. However, because of their low power and low supply voltage requirements, transconductors implementing a differential MOS transistor structure have been preferred. Currently, a CMOS gm-C filter simulating an inductance-capacitance (LC) ladder is commonly used in high-speed and high-throughput systems, such as hard-disk drives or optical disc drives.
With the ever increasing number of digital applications, one of the basic requirements for gm-c filters is for its bandwidth to be digitally (electronically) tuned—i.e., the center frequency or cutoff frequency of the filter may be adjusted electronically by the application of an appropriate control signal (e.g., tuning voltage or signal). gm-C filters are also especially useful because of their ability to be electronically (and rapidly) tuned to different bandwidth settings.
A control signal is conventionally applied to either a controllable transconductance (gm) or controllable capacitance (C) in a gm-C filter. As is well known, the transconductance of a gm-C filter may be controlled by controlling a bias current that flows in an active device, such as a bipolar or MOS (metal oxide semiconductor) transistor. The capacitance of a gm-C filter may be controlled by applying an appropriate tuning voltage to a voltage-dependent capacitance (such as a varactor diode), or by selectively switching fixed, binary-weighted capacitors. However, in high speed and high throughput systems, because of the parasitic capacitance of transistors, controlling the transconductance to tune a gm-C filter is preferred over controlling the capacitance.
A problem that commonly plagues gm-C filters is direct current (DC) offset. Because various mismatch characteristics can exist in the transistors used in a gm-C filter, in a DC coupling channel, these mismatch characteristics can make a signal non-linear, and generate an internal DC offset that can affect the true output of the gm-C filter. Additionally, because of the differential circuit structure adopted by gm-C filters, DC offset can also be propagated to the other stages in the gm-C filter, and increase across the various stages. Therefore, the overall performance of a gm-C filter can be degraded.
Current schemes to eliminate DC offset involve taking the filter offline for a duration of time and calibrating the filter to eliminate the DC offset. This process is repeated each time the filter is tuned to a different bandwidth setting. However, taking the filter offline can impose heavy restrictions on the performance of a gm-C filter especially in high-speed and high-throughput environments.